Seamless steel pipes are carbon structural steels with low hardness and easy machining. They are often used in molds as templates, stubs, guide posts, etc., but heat treatment is required. The hardness of the seamless steel pipe is higher than HRC55 after quenching and before tempering. The actual hardness is HRC55. Do not use the heat treatment process of carburizing and quenching for No. 45 steel.

Seamless steel pipes after quenching and tempering have good comprehensive mechanical properties, and are widely used in various important structural parts, especially those connecting rods, bolts, gears and shafts that work under alternating loads. But the surface hardness is low and not wear-resistant. Quenching and tempering + surface quenching can be used to improve the surface hardness of parts.

Carburizing treatment of cold-drawn seamless steel pipe is generally used for heavy-duty parts with wear-resistant surface and impact-resistant core, and its wear resistance is higher than that of quenching and tempering + surface quenching. Its surface carbon content is 0.8-1.2%, and the core is generally 0.1-0.25%. After heat treatment, the surface can obtain high hardness, the core has low hardness and impact resistance.

If No. 45 steel is carburized, hard and brittle martensite will appear in the core after quenching, and the advantages of carburizing will be lost. At present, the materials using carburizing process have low carbon content, and the core strength can reach high at 0.30%, which is rare in application.

The recommended heat treatment system of No. 45 steel cold-drawn seamless steel pipe stipulated in GB/T8162-2008 standard is normalizing at 850℃, quenching at 840℃, and tempering at 600℃, and the achieved performance is yield strength >355MPa. The GB/T699-1999 standard stipulates that the tensile strength of 45 steel is 600MPa, the yield strength is 355MPa, the elongation rate is 16%, the area shrinkage rate is 40%, and the impact energy is 39J.